Graphite is a very important material due to its unusual two dimensional anisotropic properties. It is chemically inert, electrically and thermally conducting. Graphite foil is well known in the art. Typically, graphite foil is made by several steps: graphite particles are first intercalated with acidic materials such as concentrated HNO3 and H2SO4 acids and the resulting sludge is heated very quickly to a high temperature to expand the particles; the expanded graphite phase, also called “graphite worm” is then rolled to form foil. The resulting foil is large, flexible and chemically resistant; however, the minimum thickness of commercial foil is around 5 mil (75 micrometers). Commercial foils can contain trace amounts of chlorine and sulfur remaining from the process steps which may be undesirable for some applications. During processing emission on toxic products is also an issue with the commercially available graphite foil (example, flexible graphite available as GRAFOIL, Lakewood, Ohio) may be configured to scrolls by rolling and cutting but the purity of available material and the cost are issues.
Methods of coating substrates with dry particles have been disclosed. For example U.S. Pat. No. 4,741,918 (de Nagybaczon et al.) discloses a coating process wherein dry particles are coated on a substrate with a high-energy buffing wheel. Because a buffing wheel is used, this process inherently orients the particles in the direction of travel of the buffing wheel on the substrate. Although a variety of particles are coated using the disclosed method, graphite is not among them.
U.S. Pat. No. 6,511,701 (Divigalpitiya et al.) discloses a method of coating a polymer substrate with a dry composition comprising particles. The particles have a Mohs' hardness between 1 and 2.5 and a largest dimension of less than 100 microns. The particles are buffed on the substrate with an applicator which moves in a manner parallel to the surface of the substrate. The particles can be graphite.